1. Field of the Invention
The present invention relates to a new, lightweight reflective surgical drape which is effective in reducing the rate of heat loss in human patients during a variety of surgical procedures.
2. Description of the Prior Art
Heat loss in human patients during surgical procedures often leads to intraoperative hypothermia. Such hypothermia is caused in part by anesthesia which depresses the thermal regulating centers in the hypothalamus. Also, general anesthetics and muscle relaxants block the shivering response and reduce metabolic heat production. Moreover, the use of cold, dry anesthetic gases increases evaporative heat losses in the lungs, and peripheral vasodilatation makes the patient nearly poikilothermic. In a cool operating room, reduction of a patient's body temperature to 32.degree. to 34.degree. C. (89.6.degree. to 93.2.degree. F.) is not uncommon if preventive measures are not taken. Intraoperative hypothermia is responsible for a reduction in the rate of drug metabolism, an alteration in cerebral and regional blood flow, variations in EEG recordings and increased latency to post-surgical arousal.
In general, body temperature is determined by the balance between heat production and heat loss. Euthermia is maintained by the body's ability to vary heat production and to conserve heat. An anesthetized patient, with a relatively low metabolic rate and minimal control over heat loss, is obviously at a disadvantage. Metabolic heat production in an anesthetized normal adult male is 60-70 kcal per hour. Heat is lost through four parallel pathways: conduction, evaporation, convection, and radiation. Of these, conduction and evaporation cause the fewest intraoperative problems. Conductive loss is minimal (less than 10%) because of the low specific heat and conductivity of conventional drapes and mattresses. Although evaporative heat loss (i.e., insensible perspiration plus evaportaion from the respiratory tract) is approximately 25 kcal per hour, this loss can be reduced to 10-15 kcal per hour by using moist warm-inspired gases.
The major causes of heat loss in the operating room are convection and radiation. Convective heat loss is a function of ambient temperature and the square root of air velocity. In a 21.degree. C. operating room, an exposed patient's convective heat loss can be as high as 80 kcal per hour. Conventional surgical draping reduces both the velocity and volume of air interacting with a patient's body and accordingly decreases convective heat loss to about 20 kcal per hour.
The human body is nearly a perfect emitter and absorber at the wavelengths involved in thermal exchange. Since the probability of photon reflection is nearly zero in a typical operating room, radiant heat loss is a function of the difference between the patient's body temperature and the temperature of the operating room. In a 21.degree. C. operating room, a patient's radiant heat loss can be as high as 100 kcal per hour. Accordingly, it is the rate and degree of a patient's radiant heat loss that must be reduced to prevent the onset of intraoperative hypothermia.
Changes in body temperature that lead to intraoperative hypothermia occur more frequently in pediatric patients and carry greater risks than those in adults. A sick infant is unable to maintain thermal stability and dehydration, diarrhea and weakness serve to increase heat loss. Infants on the operating table may lose considerable amounts of heat both by convection into the air-conditioned operating room and by radiation to the cool walls. The resultant low body temperature is one of the most common causes of the stoppage of breathing following general anesthesia. Frequently the infant must be rewarmed before spontaneous respiration resumes. It is therefore essential that an infant in the operating room be kept normothermic.
Unsuspected hypothermia also particularly affects the elderly, whose ability to increase heat production and to decrease heat loss by vasoconstriction in response to cold is impaired. Hypothermia in the elderly is particularly troublesome since it leads to post-anesthetic shivering (PAS). Many complications arise from PAS due to the markedly increased demand on the cardiovascular and pulmonary systems. With age, cardiovascular and pulmonary physiology decline, resulting in less reserve capacity and borderline compensated function. Therefore, particularly in older patients with generally compromised physical condition, additional care must be taken to avoid intraoperative hypothermia and the resultant PAS.
It is therefore apparent that a need exists for a viable method and apparatus for preventing intraoperative hypothermia in all surgical patients. Many different methods and apparatus including pre-warmed gel-filled mattresses, blankets with circulating warm liquid, suits with circulating warm liquid, heating lamps, radiant heaters, humidification of inspired gases and metallized plastic sheeting have been utilized in an attempt to minimize heat loss in patients during surgery.
Heat loss in infants has been conventionally minimized by keeping the infant in an incubator until the last moment, by wrapping all extremities in cotton cast padding, and by exposing as little of the body as possible during induction of anesthesia. The use of warmed, humidified anesthetic gases has also been used in preventing heat loss in infants Heat has also been supplied by placing a warming mattress just beneath the operating table cover, by increasing the operating room temperature to 24.degree. to 27.degree. C. (75.degree. to 80.degree. F.) or by performing the operation beneath a radiant heater especially when operating on premature infants.
The active methods of warming mentioned above carry the risks of overheating or burning patients while humidification of inspired gases increases the risk of bacterial or viral contamination in the breathing circuit. The use of metallized plastic sheeting is discussed below. As noted above, another conventional method of preventing heat loss in surgical patients has been to raise the ambient temperature in the operating room to 24.degree. to 27.degree. C. Surgeons, however, are most comfortable when the operating room temperature is 18.degree. C. while anesthesiologists prefer a temperature of 22.degree. C. Accordingly, this technique of preventing heat loss in surgical patients has obvious drawbacks.
The use of metallized plastic sheeting to reduce radiant heat loss was reported by Dyde and Lunn in 1970 (Thorax (1970), 25, 355). Dyde and Lunn proposed wrapping the lower half of a patient's body in a blanket of aluminum foil coated with polyethylene in an attempt to reduce heat loss during thoracotomy. Dyde and Lunn had good success in reducing heat loss in patients undergoing relatively short thoracotomy procedures.
Radford and Thurlow (Br. J. Anaesth. (1979), 51, 237) later found that the type of metallized plastic sheeting used by Dyde and Lunn was ineffective in the prevention of hypothermia in adult patients studied during neurosurgical operations. They concluded that active warming systems were needed to maintain normothermia in patients undergoing neurosurgical operations.
Radford and Thurlow used a type of metallized plastic sheeting made by Thermos under the name of "Space Blanket". Each blanket consisted of two layers of metallized plastic sheeting separated by an artificial fiber layer. Each patient in the control group wore a cotton gown and was covered by one cotton blanket. Each patient in the study group was additionally wrapped in metallized plastic sheeting. The head and shoulders arterial or venous cannula in place. No active warming system was used.
Radford and Thurlow theorized that a drawback of metallized plastic sheeting is that the infrared reflecting property of the metallic surface is reduced or lost by condensed perspiration. This theory may explain the inconsistencies in the results reported by Dyde and Lunn, and those reported by Radford and Thurlow.
Shortly after the publication of the Radford and Thurlow article one commentator observed that the insulation layer in metallized plastic sheeting is thin and that a breakdown may occur. Brit. J. Anaesthesia (1980), 52, 359. The commentator concluded that, if metallized plastic sheeting is used in conjunction with surgical diathermy (the therapeutic use of an oscillating electric current of high frequency to produce local heat in body tissues below the surface) there is a serious risk of burns from aberrant earthing. Thus, the prevailing view was that there was a significant electrical hazard present when space blankets or metallized plastic sheeting was used with diathermy and metal operating tables.
Bourke, D. L. et al. (Intraoperative Heat Conservation Using a Reflective Blanket, Anesthesiology,60: 151-154, 1984) studied the effectiveness of a reflective blanket in reducing radiant heat loss in an anesthetized patient. The reflective blanket used in the Bourke study was aluminized Tyvek, type 1443, which is used as a lining in survival apparel. All patients in the Bourke study were placed on an active heating blanket whose temperature had equilibrated with ambient temperature. The test patients were wrapped in the aluminized blanket as completely as positioning would allow. The blanket utilized in this study was perforated so that it would not trap moisture that could condense and cause skin maceration during prolonged use. The blanket utilized in this study was apparently conductive since a copper cable was used to connect the aluminized blanket to the operating table to prevent patient isolation. Also, as noted above, a perforated aluminized blanket poses a significant electrical hazard in the operating room environment. Thus, the reflective blanket utilized in the Bourke study would appear to pose a significant electrical hazard in the operating room environment.